Example #2
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EXAMPLE #2 - Balancing Redox Reactions Using the Oxidation Number Method:

Balance the following redox equation using either the inspection technique or the oxidation number method. Be sure to check that the atoms and the charge are balanced.

Cu(s) + HNO3(aq)   -->  Cu(NO3)2(aq) + NO(g) + H2O(l)

Solution:

The nitrogen atoms and the oxygen atoms are difficult to balance by inspection, so we will go to Step #3. The copper atoms are changing their oxidation number from 0 to +2, and some of the nitrogen atoms are changing from +5 to +2. These changes indicate that this reaction is a redox reaction. We next determine the changes in oxidation number for the atoms oxidized and reduced.

Cu  0 to +2      Net Change = +2

Some N  +5 to +2     Net Change = -3

We need three Cu atoms (net change of +6) for every 2 nitrogen atoms that change (net change of -6). Although the numbers for the ratio determined in Step #5 are usually put in front of reactant formulas, this equation is somewhat different.  Because some of the nitrogen atoms are changing and some are not, we need to be careful to put the 2 in front of a formula in which all of the nitrogen atoms are changing or have changed. We therefore place the 2 in front of the NO(g) on the product side. The 3 for the copper atoms can be placed in front of the Cu(s).

3Cu(s) + HNO3(aq)  -->  Cu(NO3)2(aq) + 2NO(g) + H2O(l)

We balance the rest of the atoms using the technique described in Chapter 4, being careful to keep the ratio of Cu to NO 3:2.

3Cu(s) + 8HNO3(aq)
-->  3Cu(NO3)2(aq) + 2NO(g) + 4H2O(l)

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